(1) Field of the Invention
The present invention relates to an oxygen partial pressure measuring device for determining oxygen partial pressure in exhaust gases, especially gases discharged from internal combustion engines.
(2) Description of the Prior Art
It has been known to provide conductive electrodes, such as platinum, on opposite faces of an oxygen ion conductive solid electrolytic partition made of, for example, stabilized zirconia, one electrode having a surface (first surface) adapted to be in contact with a reference atmosphere having a known oxygen partial pressure such, for example, as the air and the other electrode having a surface (second surface) adapted to be in contact with gas to be measured, for example, exhaust gas of an internal combustion engine, to provide an oxygen concentration cell at high temperatures the electromotive force of which is indicated by the Nernst equation, E=RT/nF ln P.sub.1 /P.sub.2 under ideal conditions, where E is electromotive force; T is absolute temperature; P.sub.1 and P.sub.2 are oxygen partial pressures in atmospheres contacting the first and second surfaces of the partition; R is gas constant; F is Faraday's constant; and n is charge of cation of the solid electrolyte. Such a cell could be used as an oxygen partial pressure measuring device for determining oxygen partial pressures in gases to be measured.
With exhaust gases of internal combustion engines, the oxygen partial pressure therein in a state of equilibrium is relatively high, if the excess air ratio in the exhaust gas components is more than one. However, in the event that the excess air ratio is less than one, the oxygen partial pressure is considerably low. The electromotive force of the oxygen concentration cell rapidly changes across the line of the excess air ratio one. Such a great change in electromotive force is very advantageous for controlling or maintaining the excess air ratio of the exhaust gas at one for the purpose of using the electromotive force as signals.
With the usual exhaust gases of internal combustion engines, however, they are not necessarily in the state of equilibrium because unburnt fuel gases and unreacted oxygen co-exist. In order to cause the electromotive force of the oxygen concentration cell to vary rapidly across the line of the excess air ratio one, it is required to bring the exhaust gas contacting the second surface of the cell into an equilibrium condition. Electrodes made of platinum could achieve this purpose to some extent owing to the catalytic action of platinum. However, the thickness of the platinum layer as the electrode is generally so thin that the time for the passage of the exhaust gas through the electrode layer is very short. Accordingly, catalytic action of the platinum electrode is, by itself, insufficient to bring the exhaust gas into the equilibrium condition, when the exhaust gas includes large amounts of unburnt fuel gas and oxygen. In particular, with an exhaust gas cleaning system wherein an internal combustion engine is operated at an excess air ratio less than one and secondary air is introduced into the exhaust gas to increase the excess air ratio to one, the electromotive force of the oxygen concentration cell does not change rapidly at the excess air ratio one because of the large amounts of the unburnt fuel gas and oxygen in the exhaust gas.
In order to solve this problem, it has been suggested that a porous film having a catalytic action be applied onto the second surface of the electrode to supplement the catalytic action. In this method, however, the thickness of the catalytic porous film is at the most 1 mm, so that the catalytic action is quite insufficient to bring the exhaust gas into the equilibrium condition in the event of great amounts of unburnt components in the exhaust gas, although it is effective to some extent for the exhaust gas near the equilibrium. When the thickness of the porous film having the catalytic action is made thicker in order to avoid this disadvantage, the response time will in turn become lower to an extent such that it cannot be used for practical purposes.
It has also been suggested that a catalyst carried on a granular carrier or mineral wool be arranged against the exhaust gas flow in front of the detecting end of a device so that the oxygen partial pressure is determined after the exhaust gas has passed through the catalyst to bring it into an equilibrium condition. This method complicates mounting and replacing the catalyst bed onto and from the exhaust pipe and requires a great amount of the catalyst bed which makes the head of the exhaust pipe larger and causes loss in weight of the catalyst bed due to vibrations. Moreover, the flow passages of the exhaust gases in the catalyst bed are not fixed, so that the times required for the exhaust gases to pass through the catalyst bed vary within a wide range, with the result that the composition of the exhaust gas varies for a short period of time and the output of the oxygen concentration cell does not respond sufficiently to the variation in the composition of the exhaust gas for the short period of time.